A used SCHERER machine with a Sinumerik 840D initially reported missing Siemens ET200X I/O modules and flashing BF indicators. Correcting the PROFIBUS address switches changed the symptom to PLC read Alarm 4252 and write Alarm 4254. Treat that change as evidence that addressing was only the first fault; verify the configured hardware and PLC logic before deleting blocks or bypassing inputs.
Diagnose the PROFIBUS and PLC alarms
| Observation | Engineering interpretation | Required check |
|---|---|---|
| ET200X BF flashes | The reported module address did not match the machine documentation. | Compare every physical address switch with the documented and PLC-configured PROFIBUS address. |
| Alarm 4252 after correcting addresses | A PLC read error remains. | Review machine parameters 10394, 10395, 10396, and 10397; the reported installation had all four set to 0, but the correct values for this machine are unknown. |
| Alarm 4254 after correcting addresses | A PLC write error remains. | Check the same parameters and compare the online hardware with the PLC hardware configuration. |
Do not copy parameter values from another machine merely because its displayed values are 0. Establish the intended values from this machine's project, archives, and manufacturer documentation. A correct node address does not prove that the configured module type, I/O layout, or PLC communication path matches the installed hardware.
Preserve the recoverable PLC state
Create NCK and PLC archives before changing the hardware configuration or logic, then search the control disk for older archives. Preserve the original PLC project supplied by the machine builder and archive the modified version separately. This is essential when modules or blocks have already been removed remotely.
SIMATIC STEP 7 is the identified PLC engineering software. The Sinumerik HMI may already contain STEP 7 with the corresponding 810/840D add-on; otherwise it must be installed. An external engineering computer requires the applicable MPI connection hardware described for this installation. Confirm that STEP 7 can access the PLC blocks and save an offline project before editing.
Trace missing sensors without online ET200X access
- Use the electrical drawings to identify the PLC input addresses wired to the destroyed conveyor or robot-area sensors.
- Open the offline PLC project and use STEP 7 reference data to find every read, derived condition, alarm, and interlock associated with those addresses. The configured ET200X station does not need to be online for address references already stored in the project to be searched.
- Determine whether the machine builder provided an option or configuration bit that disables the robot, conveyor, bar feeder, or other absent equipment. Prefer the designed option path over deleting complete blocks.
- If a relevant block cannot be opened, identify whether it has know-how protection or block privacy. Some protected blocks may implement Sinumerik system functions or NCK-to-PLC interaction; others may be protected machine-builder logic.
- After making an authorized change, archive the revised PLC, download it, and verify I/O status, alarm clearance, JOG operation, MDI acceptance, and Cycle Start behavior.
Changing a manufacturer menu selection from robot Active to Passive did not resolve the reported case, so that selection alone did not remove every dependency. If protected or deleted logic still generates alarms for removed options, obtain the correct machine-builder block version rather than reconstructing protected system behavior.
Resolve JOG operation with MDI Cycle Start blocked
After the ET200X modules were removed from the PLC, the machine operated in JOG but ignored Cycle Start in MDI. The remaining reported faults concerned electrical-cabinet cooling and coolant level. Do not assume those alarms are informational: the machine builder controls their severity and may use them as program-cycle permissives.
Trace each displayed alarm to its corresponding bit in interface data block DB2, then locate the logic that sets that bit and the permissive chain used by Cycle Start. Correct the cooling and coolant conditions or apply the machine builder's documented option configuration. Re-test with no critical alarms present; successful JOG movement does not prove that the MDI interlock chain is satisfied.
FAQ
What do Sinumerik 840D Alarms 4252 and 4254 indicate in this case?
Alarm 4252 was reported as a PLC read error and Alarm 4254 as a PLC write error after the ET200X PROFIBUS addresses were corrected. Check machine parameters 10394 through 10397 and compare the online hardware against the saved PLC configuration.
Can I find ET200X sensor references while the module is offline?
Yes. Identify each sensor's input address from the electrical drawings, then use STEP 7 reference data to search the offline PLC project; the remote module does not need to be accessible for stored address references to be found.
Why does Sinumerik 840D run in JOG but ignore MDI Cycle Start?
JOG and automatic-cycle permissives can differ. Trace the active cooling and coolant-level alarms through DB2 and the Cycle Start interlock logic, because the machine builder may classify either condition as critical.